docs: add a file name (#60)

I added this file name, because I was confused myself where to put this code and I wanted to clarify that

docs/content/docs/integrations/svelte-kit.mdx

@@ -8,7 +8,7 @@ Before you start, make sure you have a better auth instance configured. If you h
 
 We need to mount the handler to svelte kit server hook.
 
-```ts
+```ts title="hooks.server.ts"
 import { auth } from "$lib/auth";
 import { svelteKitHandler } from "better-auth/svelte-kit";